1. Field of the Invention
The present invention is related to etching holes in metal, particularly to etching submicron patterned holes in thin metal layers, and more particularly to electrochemical etching of submicron patterned holes in thin metal layers with the aid of a wetting agent.
2. Description of the Related Art
During the past decade, substantial research and development has been directed to fabrication of devices such as field emitters for flat panel displays, which involve the formation and etching of holes in various materials. In a number of these fabrication approaches, nuclear tracking has been utilized to form initial tracks in a mask material, after which the tracks would be etched by various techniques to form holes in one or more layers of material under the mask material.
It has long been recognized that the etching of submicron patterned holes in thin metal layers is difficult due to geometry limitations, the short duration of the mask life during etching, the adhesion of the mask to the surface of the metals, and the inability of chemical etches to wet the masking material. In the prior art, plasma etching has been used to perform the transfer process but this requires sophisticated and expensive equipment. For field emission display (FED) applications, for example, or other applications requiring submicron features patterned in metal films, the masking material is generally polycarbonate, such as LEXAN manufactured by General Electric Corporation. The LEXAN is spun on the wafers which have had a sequence of thin films deposited to form the cathode or row electrical contact, the intermetal dielectric (IMD), and the gate electrode metal. For example, the cathode is a silicon substrate, the IMD is a silicon dioxide, and the gate metal is titanium/molybdenum/chromium, with the titanium used for adhesion to the silicon dioxide surface, and the chromium used to promote the stick of LEXAN to the surface of the gate metal. For field emission display applications the cathode is a patterned row metalization, the IMD is a deposited silicon dioxide, and the gate metals could be reduced to a single metal film, such as molybdenum, chromium, or others, with a thickness on the order of 200-1000 .ANG.. After the LEXAN is spun on the processed wafer, it is baked to prepare the masking material. Practical embodiments for field emission display applications may also include a highly resistive thin film between the row metal and insulating IMD to provide resistive current limiting to any emitters exhibiting excessive field emission currents.
Holes are formed in the mask, such as LEXAN, by nuclear tracking, by implanting a low density of MeV heavy ions, such as xenon or krypton, through the mask material followed by wet etching of the nuclear tracked regions with high selectivity over the non-tracked regions. The trackable material or mask is not limited to polycarbonate or LEXAN, which exhibits the highest selectivity, but could include polyimides, polymethylmethacrylate (PMMA), or standard positive photoresists. Using a LEXAN film having etched tracks as a mask layer to transfer the patterns to the gate metal, a wafer exposed to a chlorine plasma environment both etches the patterned holes in the chromium and simultaneously removes the LEXAN. In such an embodiment, as described above, the chromium, which is only 100-200 .ANG. thick, is used as a masking material for plasma etching the molybdenum with SF.sub.6 or CF.sub.4 chemistries, after which the oxide (silicon dioxide) layer is plasma etched with CHF.sub.3 and O.sub.2 chemistry using the chromium and/or molybdenum thin layer as a mask. Field emission devices can then be formed by known techniques to form a self-aligned, gate nanofilament.
The principle problem with the prior known plasma etching scheme is the short duration of the LEXAN mask and the expensive plasma generation and vacuum pumping equipment used to perform the etch. Conventional wet chemical etching of the metal is avoided since over-etching ruins the physical structure of the hole in the metal being etched, and since conventional metal etches do not wet the LEXAN, thereby limiting both the control and uniformity for etching the structures in the gate metal.
The present invention provides a solution to the above-referenced prior art etching techniques, by providing a wet chemical process for etching submicron patterned holes in thin metal layers using electrochemical etching with the aid of a wetting agent. Basically, the process of the invention involves immersing the processed wafer in a wetting agent, and then transferring the wetted wafer to an electrochemical etching apparatus, wherein the wetting agent in the masking layer tracks is replaced by an electrolyte, after which the metal patterns exposed at the bottom of the tracks are etched by an electrochemical process, producing uniform etching of patterned holes in both the chromium and the molybdenum thin layers, utilizing the patterned LEXAN as a mask.